A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon substrate). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
European patent application EP-A-1 037 117 discloses an off-axis leveling arrangement in a lithographic projection apparatus. Using this arrangement a height map of a substrate in a lithographic apparatus is determined, by employing a level sensor using grating optics and polychromatic radiation in the wavelength range of 600-1050 nm.
European patent application EP-A-2 228 685 discloses a further level sensing arrangement wherein the light source is arranged to emit projection radiation in a wavelength range in which a resist to be used for processing the substrate in the lithographic apparatus is sensitive, for example radiation in the wavelength range of 200-400 nm.